Linked Lists: Detect a Cycle


Problem Statement :


A linked list is said to contain a cycle if any node is visited more than once while traversing the list. For example, in the following graph there is a cycle formed when node 5  points back to node 3.


Function Description

Complete the function has_cycle in the editor below. It must return a boolean true if the graph contains a cycle, or false.

has_cycle has the following parameter(s):

head: a pointer to a Node object that points to the head of a linked list.
Returns

boolean: True if there is a cycle, False if there is not
Note: If the list is empty, head  will be null.

Input Format

There is no input for this challenge. A random linked list is generated at runtime and passed to your function.

Constraints


0   <=  list size  <=  100


Solution :



title-img 




                        Solution in C++ :

In  C++  :






/*


A Node is defined as: 
    struct Node {
        int data;
        Node* next;
    }
*/

#include <unordered_set>

std::unordered_set<Node*> visited;

bool has_cycle(Node* head)
{
    // Empty lists don't have circles.
    if(head == nullptr) {
        visited.clear();
        
        return false;
    }
    
    // If already in the list of visited nodes.
    if(visited.count(head) != 0) {
        visited.clear();
        
        // It means we have a cycle.
        return true;
    }
    
    // Otherwise, remember it.
    visited.insert(head);
    
    // Recurse for the rest of the list.
    return has_cycle(head->next);
}
                    


                        Solution in Java :

In   Java :






/*


A Node is defined as: 
    class Node {
        int data;
        Node next;
    }
*/

boolean hasCycle(Node head) {
    
    
    if(head==null)
        return false;
    else{
        Node slow=head;
        Node fast=head.next;
        while(fast!=null && fast.next!=null && fast!=slow){
            slow=slow.next;
            fast=fast.next.next;
        }
        if( fast!=null && fast==slow)
            return true;
        return false;
    }    

}
                    


                        Solution in Python : 
                            
In   Python3  :






"""


A Node is defined as: 
 
    class Node(object):
        def __init__(self, data = None, next_node = None):
            self.data = data
            self.next = next_node
"""


def has_cycle(head):
    visited = set()
    it = head
    while it.next:
        it = it.next
        if it in visited:
            return True
        visited.add(it)
    return False


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